We discussed this matter at length and eventually decided that indeed the simplest approach was indeed best for now, as in all previous studies of this type. We decided that incorperating respiratory tissue density would require multiple dissections of archosaurs lungs etc., and it was suggested to us by avian respiratory biologists in our lab that this would not be a trivial undertaking, and ultimately, as Andy says, it would add more speculative assumptions when it comes to the dinosaurs. They suggested that there might actually be density data already published (on pigeons and/or magpie) but I couldnt find it. Definitely something to think about though for future studies, particularly as more details on extant archosaur respiratory anatomy are published (see Claessens et al this week in PLoS ONE), and is something definitely worth doing if the next stage of analysis involves inferences of repiratory capacity.